Cholecystokinin-8-sulfate (CCK-8 sulfate) injected intrathecally antagonizes the morphine induced increase of tail flick latency in rats. The mechanism underlying this action of CCK-8-sulfate against morphine analgesia was investigated in this study. The CCK-8-sulfate antagonism of morphine analgesia was greatly reduced when rats were pretreated with proglumide, a cholecystokinin receptor antagonist. By using rats implanted with push-pull cannulae at spinal cord, the action of morphine on CCK release from spinal cord was tested. When subarachnoid spaces of the rat spinal cord was infused with morphine containing medium, efflux of CCK-like immunoreactive material was detected. This increased efflux of CCK-like material was reduced greatly by naloxone. The result seems to suggest that endogenous CCK may be important in development of morphine tolerance. Therefore, the effect of proglumide on development of morphine tolerance was tested. The tolerance to analgesia induced by repeated injections of morphine was markedly reduced when proglumide was given in combination with morphine to the rat. FMRF-NH2 also antagonizes the tail flick latency increase elicited by morphine and this action is again antagonized by proglumide. Whether endogenous FMRF-NH2 participates in development of morphine tolerance will be further investigated.